It is known in the prior art to create sealed cavities on an integrated circuit wafer for a variety of applications, for example, as a pressure sensor or a microphone. It is also known to encapsulate movable micro-mechanical components on a wafer within a sealed cavity. The encapsulation of micro-mechanical structures in a sealed cavity is desirable for several reasons. First, a seal that substantially prevents water vapor, dust, and atmospheric gases from entering the space adjacent to the mechanical structure greatly improves the tolerance of the micro-mechanical structures to ambient conditions, such as high humidity. Second, the dicing and packaging of the wafers bearing micro-mechanical structures is greatly facilitated because standard water-based saw slurries can be used without concern that the slurry will contaminate the micro-mechanical structures. Third, when the sealed cavity is at a low or very low pressure, the Brownian noise due to the motion of gas molecules can be significantly reduced.
Processes for the creation of sealed cavities containing MEMS devices using thin-film depositions are well known in the prior art. For purposes of this application, the term “thin film deposition” refers to any deposition scheme in which the atoms are assembled from a gaseous or plasma phase onto the surface of the wafer bearing the micro-mechanical device(s). See, for example, U.S. Pat. Nos. 5,285,131 and 5,493,177 (both to Muller, et al.) in which methods to create an incandescent lamp and a vacuum tube respectively are disclosed. The use of thin-film deposition techniques is desirable, because of the accuracy with which the thin films can be patterned, the high quality of the adhesion of the thin films to the wafer surface, and the low cost involved in thin film deposition techniques, compared to wafer-wafer bonding approaches.
A typical thin-film deposition process is as follows. A silicon substrate is covered with a protective layer that is selectively removed, thereby exposing the silicon wafer in the region to be encapsulated. Then, a layer of sacrificial material is deposited to support the structural layer as it is formed. The structural layer is deposited and patterned on the sacrificial layer, and is then covered by a second sacrificial layer. The second sacrificial layer supports a cap layer during its formation. To remove the sacrificial layers, thereby releasing a MEMS or other micro-mechanical device, holes are etched through the cap layer down to the sacrificial layers and an etching agent is introduced to remove the sacrificial layers. Once the MEMS device has been released, the holes in the cap layer are sealed by another thin-film deposition process. Complex structures may require additional layers of structural or sacrificial materials.
Typically, the cap layer is composed of a thin layer of silicon nitride. However, although silicon nitride is a very hard material, it is difficult to deposit silicon nitride in thick layers with good control of stresses. Alternatively, caps have been made of metal, such as aluminum. In this case the cap thickness can be made greater; however, the material itself is ductile and can be deformed by pressure. Therefore, one problem with the capping technology that is available today is that the caps are relatively weak. Differences in pressure created when the sealed cavity encloses a vacuum can cause the cap to collapse inward. Additionally, stresses placed on the cap during packaging can cause the cap to collapse or to bow inward. For example, enclosing the wafer in plastic packaging exposes the cap to hot, high pressure plastic, often at temperatures up to about 300 degrees C., and pressures up to about 3000 psi, during the injection molding process. Such conditions can often damage these caps, resulting in the destruction of the fragile encapsulated microstructure. Caps constructed using the thin-film deposition technologies available today are not able to withstand such pressures.
To avoid damage during the packaging process, it is known in the prior art to cover micro-mechanical structures on a wafer with a second wafer made of silicon or glass, which has an etched cavity over the micro-mechanical structure, and which is in some way bonded to the original wafer bearing the micro-mechanical structures. Several methods are well known in the art for creating a bond between the wafer containing the micro-mechanical structures and the wafer that implements the cap. In particular, anodic bonding can be used. In addition, by depositing and patterning a eutectic metal alloy in the region where a seal is to be formed, the two wafers can be bonded in a process that is much like soldering. However, all of these two-wafer methods, in addition to adding significant expense to the processing thereof, reduce the available device area on the surface of the wafer due to space needed to support the capping wafer seal to the micro-mechanical wafer, thereby resulting in fewer devices per wafer and increasing the cost per device.
Therefore, it would be desirable to introduce a way to strengthen a thin-film cap to encapsulate a micro-mechanical device such that it is not susceptible to damage caused by the difference in pressure between the sealed cavity and the ambient or by the harsh environment that it may be exposed to during the plastic injection molding process, and which also conserves wafer real estate.